Due to growing demand of reducing the logistic costs in food packaging industry, glass and metal packaging materials have been replaced by plastic materials in recent few years. The advantage of using plastic material are manifold such as reductions in weight and cost, and lower risk of breakage, however the barrier properties of these materials are invariably different from glass and metal packaging materials, that further impacts the shelf stability of the product.
A major concern for many products, and particularly for beverages like beer, is oxidation degradation by oxygen ingress causing taste changes and darkening of the beer, flattening of taste by carbon dioxide loss and damage due to UV light. Over the planned shelf life of a bottle about 1 ppm oxygen maximum ingress into the bottle is acceptable. The egress of carbon dioxide from the beverage through the bottle walls also has to be attenuated to a minimum. For successful conversion from glass to plastic it is very essential to consider oxygen, carbon dioxide, water vapor and flavor scalping as they affect the quality of the products particularly when stored over extended periods of time.
Polyethylene terephthalate polyesters are widely used for fabricating various components such as fibers, sheets, tubes and container owing to their superior mechanical, thermal and gas barrier properties, chemical resistance, flavor-retaining property, transparency, hygienic property, easy of processability, recyclability, and suitability for food contact applications etc.
Polyethylene terphthalate, because of its comparatively better thermal, mechanical, optical and barrier properties, is preferred over other polyesters in packaging applications e.g. use of PET in making beverage container, beer keg, etc. Therefore, Polyethylene terphthalate resins are widely used in the food packaging industry to manufacture bottles and films. PET bottles are used for the carbonated soft drink, fruit juice and mineral water. These products have a shelf life of 8-12 weeks and over this period the gas permeability properties of PET are considered sufficient. However, alcoholic beverages like beer are much more sensitive to oxygen and carbon dioxide diffusion either into or out of the bottle. When this sensitivity to migrating gases is combined with the need for a longer shelf life, it is necessary to improve on the gas permeability properties of PET.
Replacing the glass and metal packaging by polyethylene terephthalate based material is very tedious task that requires maintaining good barrier, optical, thermal and mechanical properties of the bottle so as to improve the shelf life of the food products being packaged.
Various technologies as to making of PET based containers for food packaging exists in the market. Containers can be made from non-PET polyesters as well with lower gas permeability, such as polyethylene naphthalate (PEN). However, there are difficulties in stretch blow molding these bottles and the technology has achieved limited success due to erratic behavior and haze in injection molding with conventional technology. Further difficulties are encountered in bottle blowing of PEN polymer or its alloy/blends with PET. On the whole, PEN containers are also very expensive.
Mono layer polyethylene terephthalate bottles made from PET blended or compounded with naphthalates and isophthalates, such as terpolymer or TIN polymer, have also been attempted with no significant success.
A few patents viz. U.S. Pat. No. 8,124,202, U.S. Pat. No. 6,355,738, and US200602446245 disclose use of PET in combination of other polyesters or additives to make monolayers or multilayers containers with good barrier properties and thermal properties.
US20060182911 patent application discloses blends of barrier resins consisting of a composition of Polyethylene terephthalate and polytrimethylene naphthalate, or polyethylene terephthalate and polybutylene naphthalate exclusively for packaging beverages like beer in a monolayer bottle outperforming the existing other barrier multi-layer bottles is described. The blends of PET/PTN and PET/PBN are produced by using in situ polymerization or melt blending the two polymers or compounding the two polymers to get the PTN and PBN in a PET polymer matrix. The composition of PET blends with PTN and PBN additionally contains other barrier improving additives.
However, the polybutylene naphthalate (PBN) used above is a normal PBN that does not reflect required optical properties to achieve excellent transparency of bottles or containers made from blend thereof as PBN has opaque nature. Therefore when normal PBN is used as such in manufacturing polyester containers, it creates haze in the containers. In order to overcome this problem there is a need to use modified PBN having excellent transparency to avoid haze in the containers.
There is still need to modify polyethylene terphthalate by using modified polybutylene naphthalate to achieve excellent transparency and improved barrier properties of the polyester materials.
The present invention overcomes the foregoing problem by providing a modified polyethylene terphthalate for use in making containers both in monolayer and multilayer construction with improved thermal, mechanical, optical and superior barrier properties.
Thus, modified copolyester becomes usable in packaging application for making containers with enhanced gas barrier properties, longer shelf life and and further recycled in PET stream.